Palladium complexes hydrocyanation

Because of its low acidity, hydrogen cyanide seldom adds to nonactivated multiple bonds. Catalytic processes, however, may be applied to achieve such additions. Metal catalysts, mainly nickel and palladium complexes, and [Co(CO)4]2 are used to catalyze the addition of HCN to alkenes known as hydrocyanation.l67 l74 Most studies usually apply nickel triarylphosphites with a Lewis acid promoter. The mechanism involves the insertion of the alkene into the Ni—H bond of a hydrido nickel cyanide complex to form a cr-alkylnickel complex173-176 (Scheme 6.3). The addition of DCN to deuterium-labeled compound 17 was shown to take place... 

The polymer of methyl methacrylate (MMA) is known as Perspex. It is a clear transparent glasslike material with high hardness, resistance to fracture, and chemical stability. The conventional route, as shown by reaction 4.10, involves the reaction between acetone and hydrocyanic acid, followed by sequential hydrolysis, dehydration, and esterification. This process generates large quantities of solid wastes. An alternative route based on a homogeneous palladium catalyst has recently been developed by Shell. In this process a palladium complex catalyzes the reaction between propyne (methyl acetylene), methanol, and carbon monoxide. This is shown by reaction 4.11. The desired product is formed with a regioselectivity that could be as high as 99.95%. 

Olefin hydrocyanation using palladium catalysts has been less well studied than with nickel. Nevertheless, zerovalent complexes of palladium, particulrly triarylphosphite complexes, hydrocyanate a wide range of olefins in useful yields (see Table 1). Early work reported the merit of excess phosphorus ligand to promote the reaction, and further paralleling the observations with nickel, Lewis acids have been used to improve catalytic activity. However, addition of ZnClj fails to improve nitrile product yield . Asymmetric induction in hydrocyanation results in optical yields of 30% in the synthesis of exo-2-cyanonorbomane using the chiral ligand DIOP, and studies on the stereochemistry of HCN and DCN addition to terminal alkenes and a substituted cyclohexene with the same catalyst have been reported. ... 

Mechanistic studies on the reductive elimination of square-planar type aryl(j7 -allyl)palladium complexes demonstrated occurrence of bond formation between the aryl carbon and one of the allyl termini that are located cis to each other (Scheme 8.53) [91]. The allyl ligand remained 17 -coordinated during the coupling. Similar reductive elimination between 17 -allyl and cyano ligands may be a key step in the industrially important nickel catalyzed hydrocyanation of dienes (Scheme 8.54) [92]. 

Some metal complexes of metals other than nickel are known to catalyze the hydrocyanation of alkenes, among the best being those of palladium and cobalt. ... 

Even in the early days of homogeneous hydrocyanation the reaction of norbor-nene with hydrogen cyanide in the presence of tetrakisftriphenyl phosphite)palla-dium(O) 12 indicated the influence of steric factors, since exo-5-cyanobicy-clo[2.2.1]heptane (Structure 10) is obtained stereospecifically. This result was confirmed in similar reactions showing that the entering cyano group is directed into the exo-position of the norbomene system [28]. This is due to the complexa-tion of the palladium(O) center to the exo-face of norbomene. Recent experiments have also utilized the bicyclic system to demonstrate asymmetric hydrocyanation induced by chiral palladium diphosphine complexes. Depending on the applied ligand system 11-17, an enantiomeric excess (ee) up to 40% is obtained [25]. 

Ditertiary phosphines such as (86), (92), and (98) (100) (Scheme 6) have found important uses as ligands for metal-catalyzed transformations, including e.g., palladium-catalyzed Grignard cross couplings,194,205 rhodium-catalyzed Michael additions,2 hydrocyanations,206 copolymerizations,20 and palladium-catalyzed animations.208 Rhodium complexes of (86) are catalysts for the carbonylation of methanol.188 More recently the ligand bite angle of ditertiary phosphines such as (100) has been shown to influence catalytic activity/selectivity in several important catalytic processes.209-213... 

The coordination chemistry of many chiral ligands is directed towards preparing complexes with catalytically useful metals, most notably late transition metals. Some Rh, Pd, Pt, and Ag complexes of Diop (149) (and also (150)) have been described,327,328,381,382 along with the use of palladium Diop complexes in asymmetric palladium catalysed cross-couplings383 and alkene hydrocyanation reactions. 

These results coupled with the greater success of nickel and palladium catalysts may explain the long interval preceding the next report on cobalt catalysts . Cobalt(I) complexes (see Table 1) are, however, effective hydrocyanation catalysts, not only in hydro-cyanating olefins such as 4-pentenenitrile, but also are effective for isomerizing 2-methyl-... 

The mechanism of the related palladium-catalyzed asymmetric hydrocyanation of alkenes has also been examined in detail using [Pd(RR-diop)(C2H4)] and related chiral species as precursor complexes. These additions gave the exo nitrile product with up to 40% enantiomeric excess, indicating stereoselective complexa-tion of norbornene to the Pd(0) via the exo face. The plausible reaction intermediates [Pd(diop)(norbornene)] and the hydrido cyanide (60) were characterized by and P NMR spectroscopy. Oxidative addition of HCN to Pd(0) is believed to precede rate-determining alkene binding. 

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